Modulation systems are known from the state of the art for modulating a first signal based on a second signal, for instance for modulating a radio frequency carrier based on information that is to be transmitted in a cellular system.
Due to the increasing requirements on spectral efficiency, variable envelope modulation methods have become more and more popular in cellular systems. With variable envelope modulation methods, however, the limited efficiency of conventional power amplifiers may cause heat and/or operation time problems in the transmitter equipment, in particular in thermally limited equipment like terminals. In order to reduce the amount of these effects, new transmitter architectures making use of switching mode power amplifiers have been proposed. Switching mode power amplifiers can theoretically approach a power efficiency of 100%. They have the disadvantage that they are extremely amplitude non-linear, but they do not alter significantly the phase of the input phase modulated signals.
A modulation system which enables a variable envelope modulation while making use of switching mode power amplifiers is the LINC (Linear Amplification with Nonlinear Components) system, which was proposed by D. C. Cox of the Bell Laboratories in “Linear Amplification with Nonlinear Components”, IEEE Transactions on Communications, COM-22, pp. 1942 to 1945, December 1974. The basic principle of the LINC system is to represent any arbitrary bandpass signal, which has both amplitude and phase variations, by means of two signals which are of constant amplitude and have only phase variations. These two angle modulated signals can be amplified separately using power efficient nonlinear amplifiers. The outputs of these amplifiers are then combined by a summing unit in order to produce the desired variable amplitude signal. A problem of this system is the combination of two non-coherent amplified signals. The efficiency of the system varies with the momentary angle between the two amplified signals and will thus be below 100%.
In a pulse-width modulation (PWM) system, the original signal is coded to a two-level signal that has pulses of varying widths. The mean value of the two-level signal follows the desired output signal and can be extracted by filtering. Pulse density modulation (PDM) is an alternative way to accomplish the same task. These methods are widely used for switching mode power supplies.
A PWM system has been described for example in document EP 1 271 870 A2, which presents also a possibility of creating a three-level bandpass PWM signal instead of the traditional two-level low-pass PWM signals only.
Another pulse modulation method, which resembles the above-described PWM systems, is enabled by a Sigma-Delta modulation system. A bandpass Sigma-Delta modulator can also be employed for transforming a modulated sinusoidal carrier to a two-level signal.
It is desirable, however, to provide alternative solutions for coding an original control signal to a two-level signal.